Shear flow joints, which are very commonly used in aircraft, include two or more members such as metal sheets that are held together by fastening devices which include a pin that passes through aligned holes in the members. The shank of the pin has adequate strength of material and cross-section area to resist and transfer the shear forces exerted on the joint. These forces are often quite large. The fastening means must also be able to resist the axial tensile forces developed as a consequence of bending of the joint. For this purpose, the pin has a head and a peripheral groove, and a fastener can be applied to the groove so the joint members are clamped between the head and the fastener. These forces are generally considerably smaller than the shear forces.
It has been the usual practice to form the head and groove in a conventional configuration, with the result that the head and grooved sections are too strong and heavy. This has a dual penalty. First, excess weight is undesirable in any flying device, and second, materials such as titanium are so expensive that the unnecessary material is not truly affordable.
It is an object of this invention to provide a pin for this type of joint in which the head and groove strengths are substantially equal, and only strong enough to resist the axial tensile forces generated by bending in the joint. On the other hand, the shank of the pin can be made as large as desired in order to transfer the applied shear load. Thus, a balanced pin is provided which has no excess weight, and which does not waste material.
It is another object of this invention to prepare the pin in such a way that it optimally resists fatigue forces at a critical region near the end of the cylindrical shank.